One of the most difficult technical challenges for Hit-to-Kill Missile Defense Systems is target discrimination. Nations that have a technology base to produce missiles and warheads will be capable of some level of countermeasures. Also launch and deployment debris contribute many objects to the threat cloud.
In current missile defense systems the bulk of this discrimination task falls on surface based radar supported by overhead assets. The primary radar discrimination methods are dependent on object trajectory and dynamics. The addition of an angle, angle, range (AAR) laser radar or LADAR to an IR seeker brings a number of possibilities for discrimination, robust tracking in the presence of countermeasures and clutter, accurate aim point selection and others including the use of high resolution tracks of unresolved objects to increase the probability of correct discrimination. To accomplish this one needs to be able to resolve closely spaced objects. To do this one needs to obtain precise target dynamics to be able to separate a remote reentry vehicle from chaff or other counter measures it deploys. This is accomplished through a technique known as stripping in which characteristics of all of the targets are measured and analyzed.
Stripping relies on atmospheric drag to separate objects based on their ballistic coefficient β=M/CdA where M is the object mass, Cd is the coefficient of drag and A is the object area. Typical decoys are only a few percent of the RV mass and their area is by design a match, or more likely a distribution about the reentry vehicle (RV) area. Surface based radars use stripping, trajectory and object dynamics (spin and precession) to discriminate the threat. Due to the coarseness of radar measurements a significant atmospheric drag is required (50 to 70 km altitude) to produce a measurable amount of relative velocity or separation. The density of objects and the deployment of chaff complicates or delays extraction of target dynamics and subsequent discrimination. Also the measurement accuracy of the radar is dependent on the signal-to-noise ratio (SNR), which can contribute to late discrimination. Early discrimination is desirable for the following reasons: appropriate interceptor commitment to threat load, increased range of destroyed warhead from defended assets, and enabling a shoot-look-shoot engagement. While surface radars can be used for stripping, their use for targets at extreme high altitudes would have seemed to be of limited value due to the thinness of atmosphere at these altitudes.
By way of further background, automatic target recognition is described by Sims et al. in U.S. Pat. No. 6,042,050 in which passive infrared imagery is utilized in combination with LADAR.
While this Sims et al. patent describes the use of IR and laser range finding capabilities, no stripping is indicated for high altitude targets. Nor is a combined infrared sensor and LADAR range sensor provided with one sensor boresighted on the other. In fact the LADAR in the Sims et al. patent is used to differentiate terrestrial man made targets such as tanks from natural background and does not address the problem of discrimination against highflying decoys deployed by reentry vehicles. Other U.S. Patents such as U.S. Pat. No. 6,323,941; and U.S. Pat. No. 5,345,304 describe multi-mode sensors for both LADAR and IR.
U.S. Pat. Nos. 6,082,666; 3,982,713; and 5,611,502 describe missile defense systems, whereas U.S. Pat. No. 4,471,358 describes the use of chaff for protecting a ballistic missile from detection Other U.S. Patents relating to tracking and decoy resolution are U.S. Pat. Nos. 4,308,538; 5,340,056; 5,082,211; and 5,806,801.